HOOKLESS1 (HLS1) is an ethylene induced gene required for apical hook formation in Arabidopsis. It encodes a putative acetyltransferase, which has been shown to conjugate an acetyl group to a broad range of substrates, including small molecules such as spermadine and serotonin, the N-termini of proteins, and specific lysine side chains in histories. hlsl mutants completely lack an apical hook, but are otherwise wild type for ethylene-induced morphological changes, suggesting that it regulates differential cell growth and cell expansion in seedlings in response to the plant hormones ethylene and auxin. However, it is unclear how HLS 1 affects these processes at the molecular level. I propose to dissect the role of HLS 1 by determining its subcellular localization, targets and interacting factors. GFP and HLS 1 fusion genes will be expressed in plant cell to determine the subcellular localization of the protein. The yeast two-hybrid system and a biochemical chromatography method will be used to identify HLS1-interacting proteins. Suppressors of hls1 mutant, which restores the apical hook structure, will be screened. A screen using high density arrays of full length cDNA will be carried out to identify genes that require HLS1 for induction or repression. Finally, functions of HLS 1 homologs in plant growth and development will be investigated.